The invention relates to a brakings system for a road vehicle having a drive-slip control device (ASR) and an anti-lock system (ABS) and with a hydraulic dual-circuit brake system, in which one brake circuit is assigned to the driven vehicle wheels and the other brake circuit to the non-driven vehicle wheels. The two brake circuits are designed as static brake circuits, with each circuit being assigned to an outlet-pressure space of a brake unit provided for brake actuation. The braking system operates with the following features:
a) At least on the brake circuit of the driven vehicle wheels, the ABS works on the return-flow principle, whereby in pressure reduction phases of the anti-lock control that quantity of brake fluid (corresponding to that quantity of brake fluid flowing off from the wheel brake(s) into a return line connected to a buffer accumulator) is pumped back into the brake unit by a return pump which is connected on its inlet side to the buffer accumulator and on the outlet side to the brake unit; PA1 b) The ASR works on the principle of decelerating a vehicle wheel tending to spin, by subjecting its wheel brake to pressure from an auxiliary-pressure source in such a way that its drive slip remains within a value range compatible with sufficient dynamic stability of the vehicle; PA1 c) For the drive-slip control mode, the return pump of the ABS (which is assigned to the brake circuit of the driven vehicle wheels and which has its inlet side connected to a return line of the brake circuit of the driven vehicle wheels for directing brake fluid from the reservoir to the return pump during ASR) is also utilized as an auxiliary-pressure source from which, during pressure build-up phases, pressure is supplied to the wheel brake of the vehicle wheel to be decelerated during ASR; PA1 d) An ASR control valve is changed over from a basic position O (which is assigned to the normal braking mode and ABS mode and in which brake fluid can be positively displaced into the wheel-brake cylinders of the brakes of the driven vehicle wheels as a result of the actuation of the brake unit) into an alternative functional position I, in which flow-off of brake fluid from the main brake line of the brake circuit of the driven vehicle wheels to its outlet-pressure space in the brake unit is prevented; PA1 e) An accumulator isolating valve is located in a return line of the brake circuit (II) of the driven vehicle wheels and is shut off from a buffer accumulator and from an inlet of the return pump during normal braking, and is connected to the buffer accumulator during ASR;
An ASR of this general type is the subject of the applicant's own older and not previously published German Offenlegeungsschrift 3,802,133 which describes, in combination with an ABS working on the known return-flow principle, an ASR working on the principle of decelerating a driven vehicle wheel tending to spin by subjecting its wheel brake to pressure and wherein a return pump of the ABS assigned to the brake circuit of the driven vehicle wheels is utilized as an auxiliary-pressure source for the ASR. This return pump, conventionally designed as a free-piston pump, is fed via an outlet non-return valve of precharging pump during ASR to direct brake fluid from out of the brake-fluid reservoir of the brake system into the return line of the ABS. For generating brake pressure, a return pump conveys brake fluid into the main brake line which is shut off from the brake unit by means of an ASR control valve during ASR. Furthermore, there is an ASR outlet valve which is connected between the return line of the brake circuit of the driven vehicle wheels and the brake-fluid reservoirs. The basic position of the ASR outlet valve is its shut-off position and only in pressure reduction phases of ASR is it moved into its throughflow position so that brake fluid can flow off from the return line towards the brake-fluid reservoir.
In normal braking, that is to say braking not undergoing a brake-pressure control, this return line is pressureless. Consequently, a leak in the admission-pressure supply path of the return pump utilized for the drive-slip control, (for example a leak of the outlet non-return valve of the precharging pump or of the ASR outlet valve), cannot be detected.
However, if such a leak is present and the front-axle brake circuit fails during a braking operation, then as a result of this (if during this braking operation the ABS responds on the rear-axle driven vehicle wheel brake circuit) this brake circuit can empty via the leak, with the consequence that the two brake circuits fail. This complete failure of the brake system is permanent. The potential danger that with a failure of the front-axle brake circuit, the rear-axle brake circuit too will fail, (if the latter has a leak in the return-pump supply circuit) is of course unacceptable. This is especially true since the probability of failure of the rear-axle brake circuit is relatively high because it opens the ASR outlet valve towards the brake-fluid reservoir and because the precharging pump is connected to the return line.
The object of the invention is to improve an ASR of the type aforementioned to the effect that a leak in the admission-pressure supply circuit of the return pump is clearly detectable by pedal reaction during the actuation of the brake system, even during normal braking, and the danger of a complete failure of the brake system can thereby be countered in good time.
According to the invention, this object is achieved by having an accumulator isolating valve in a return line of the brake circuit of the driven vehicle wheels shut off from the buffer accumulator and from the inlet of the return pump during normal braking and which can be connected to the buffer accumulator during ABS. An outlet non-return valve is connected between the return line and the brake-fluid reservoirs and is held in its shut-off position by means of a higher pressure in the return line. At least one isolating valve is opened to connect a portion of the low-pressure supply path of the return pump extending from the outlet non-return valve to one of the wheel brakes in the functional position assigned to normal braking. The low-pressure supply path of the return pump is shut off from the wheel brake(s) in the functional position assigned to a pressure build-up phase of the drive-slip control. An accumulator isolating valve is provided in the return line of the brake circuit of the driven vehicle wheels so that the return line can be shut off from its return pump and from a buffer accumulator which is connected to the inlet side of the return pump. In the brake-pressure reduction mode of ABS, the accumulator isolating valve opens to allow brake fluid to flow from the wheel brakes (subjected to ABS) to the return pump. Before this flow is pumped back into the main brake line of this brake circuit by the return pump, an activation of valve(s) provided for normal brake-pressure control for the brakes affected by ABS occurs. These valves are changed from their functional position assigned to the normal braking mode wherein the brakes are connected to the brake unit to a position closing the connection and the affected wheel brake(s) is connected to the low-pressure supply path of the return pump. In the functional position of these valves assigned to the pressure build-up phases of ASR, the low-pressure supply path of the return pump is shut off from the wheel brake(s) of the brake circuit of the driven vehicle wheels, by means of only a single additional solenoid valve. This ensures that even during normal braking, the safety-related functional elements, namely the outlet non-return valve of the precharging pump and, if appropriate, the ASR outlet valve, are subjected to brake pressure from the brake unit on one side and therefore a leak of these elements can be detected immediately by means of the pedal reaction (the yielding of the brake pedal). This gain in safety achieved is considerable.
For ASR, an ASR control 2/2-way control valve is provided to shut off the main brake line of the brake circuit (II) of the driven vehicle wheels between the brake-pressure regulating valves of the wheel brakes and the brake unit. The basic position O of the ASR control valve its throughflow position and its functional position utilized for ASR control is its shut-off position.
Alternatively to this, a solenoid valve is directly activatable by output signals from an electronic ASR and ABS control unit, which generates activating signals necessary for the activation of the ASR control valve, for the brake-pressure regulating valves and for further solenoid valves of the ASR and of the ABS.
As another alternative, the ASR control valve can be designed as a mechanically controlled valve which, at the onset of the drive-slip control, is changed over to its shutting-off functional position and which otherwise assumes a basic throughflow position intended for braking. Here the mechanical ASR control valve is acted on indirectly, for example as a result of the activation of the precharging pump, the operation of which is controlled by output signals from the electronic control unit.
An advantageously simple design for the mechanical actuation of the ASR control valve can be obtained by a hydromechanical control slide with an actuating piston. The slide is displaceable in a housing and is urged by a restoring spring into its basic open position, providing communication between the outlet-pressure space of the brake unit (assigned to the brake circuit of the driven vehicle wheels) and the main brake line. The piston is equipped with a piston flange which forms with the control-slide housing, a one-sided axial movable limitation of a control-pressure space. The space is subject to the outlet pressure of a precharging pump provided for supplying the return pump. The actuating piston experiences a displacement when connected to the outlet pressure, as a result of which, the ASR control valve assumes its functional closed position to shut off the brake unit from the main brake line of the brake circuit of the driven vehicle wheels. The slide is held in this displaced position as long as the precharging pump is activated.
If there are two isolating valves, one for each of the wheel brakes, each of the wheel brakes can individually or jointly be shut off from or connected to the return line of the brake circuit of the driven vehicle wheels. This affords an increased safety in terms of a checking of the leaking of the functional elements of the ASR.
If these isolating valves are designed as 2/2-way valves which can be activated by output signals from the electronic control unit provided for controlling the anti-lock and drive-slip control phases, then they can act in combination with simple 2/2-way solenoid valves as brake-pressure control valves (both for controlling anti-lock control operations and for controlling drive-slip control operations). This provides a "decoupled" control, in such a way that brake pressure is built up on one wheel brake of the brake circuit of the driven vehicle wheels and brake pressure is reduced on its other wheel brake.
In contrast, if the isolating valves are only jointly activatable and only one further brake-pressure regulating 2/2 way solenoid valve is provided for each of the wheel brakes of the brake circuit of the driven vehicle wheels, then only a "coupled" control is possible during both ABS and ASR. In such an arrangement, changes in brake pressure can be made on one wheel brake at a given time and the other wheel brake normally maintains the hitherto activated brake pressure.
However, if two isolating valves are designed as hydraulically activatable valves which are changed over from their shutting-off basic position O (suitable for the drive-slip control mode) into their throughflow position I (suitable for the non-controlled or controlled braking mode) by outlet pressure of the brake unit, the isolating valves can be jointly activated. They can be designed as mechanically activatable valves which, in comparison with solenoid valves, are of substantially simpler construction and are correspondingly cheaper.
Constructively simple alternative versions for joint actuation of the isolating valves can be obtained as follows. If, as a result of the displacement of a piston of the brake occurring during an actuation thereof, the two isolating valves can be changed over from their shut-off ASP position (O) into their throughflow positions (I) for normal braking mode and for ABS braking, the change-over can take place within a small fraction of the possible total stroke of the brake-unit piston. Alternatively a precharging pump can be provided for supplying brake fluid to the return pump, wherein the two isolating valves are actuatable by means of a hydromechanical control slide. The slide is equipped with an actuating piston which is displaceable in a slide housing and is urged by a restoring spring into its basic position. Here the piston operates the two isolating valves into their basic throughflow ABS position in which the wheel brakes of the brake circuit of the driven vehicle wheels are connected to the return line of this brake circuit. The slide is also equipped with a piston flange, which forms with the control-slide housing, a one-sided axially movable limitation of a control-pressure space. The outlet pressure of the precharging pump is permanently connected to this control-pressure space to cause the actuating piston to be displaced to actuate the two isolating valves into their shut-off ABS positions and to remain held in this functional position as long as the charging pump is activated.
An ASR outlet valve actuated in response to pressure in a line subject to the brake pressure during normal braking or during ABS braking and belonging to the brake circuit of the drive vehicle wheels, can be put in communicating connection with the pressureless brake-fluid reservoir of the brake system for the duration of the pressure reduction phases of the drive-slip control. Otherwise the ASR outlet valve is shut off from the brake-fluid reservoir. This arrangement can be utilized for controlling brake-pressure reduction phases of the drive-slip control and can be inserted into the hydraulic unit of the ABS and ASR in the alternative hydraulic circuit versions such a as: Having ASR outlet valve connected between the return line and the brake-fluid reservoir of the brake system, or connected between the main brake line of the brake circuit assigned to the driven vehicle wheels and the brake-fluid reservoir.
If, the ASR control valve is connected between the main brake line of the brake circuit assigned to the driven vehicle wheels and the brake unit, then the brake-pressure regulating valves can be designed as 2/2-way solenoid valves of simple construction which can be used both as inlet and as outlet valves.
Alternative versions for the design and activation of the ASR outlet valve is obtained when the ASR outlet valve is designed as a solenoid valve which is activatable by means of output signals from the electronic control unit provided for the ABS and the ASR. Alternatively, when a precharging pump is provided for supplying the inlet of the return pump, the ASR outlet valve is designed as a mechanically switchable 2/2-way valve for actuation by a hydromechanical control slide with an actuating piston which engages an actuating member of the outlet valve. The slide is urged by a restoring spring into its basic position in which the ASR outlet valve assumes its basic position O to shut off the return line of the brake circuit of the driven vehicle wheels from the pressureless reservoir of the brake system. Upon subjecting a control-pressure space of the control slide to the outlet pressure of the precharging pump, the slide experiences a displacement and thereby actuates the outlet valve into its functional position (I) connecting the return line to the reservoir.
In this last version there is a first supply control valve for continuing flow from the precharging pump into the pump chamber of the return pump of the brake circuit of the driven vehicle wheels. Simultaneously actuation of the supply control valve with the ASR outlet valve and the ASR control valve can change the position of the supply control valve from its basic position (O), in which the return line is connected via a low-pressure supply line to the outlet side of the outlet non-return valve of the precharging pump, into a functional position (I) in which the outlet side of the outlet non-return valve of the precharging pump is connected directly to the inlet non-return valve of the return pump. A second supply control valve has a basic closed position during a braking mode to shut off the precharging pump from its outlet non-return valve. This second supply control valve is changed over to its functional throughflow ASR position I only after the first supply control valve, the outlet valve, and the ASR control valve have reached their functional ASR positions. This last alternative allows for a low-pressure supply circuit for the return pump of the brake circuit of the driven vehicle wheels. This design is especially favorable in functional terms and ensures that both the ASR control valve and the ASR outlet valve have assumed their functional positions, necessary for the drive-slip control mode, before brake pressure can be built up in a wheel brake subjected to the drive-slip control.
In combination with this, it is advantageous if the second supply control valve can be changed over by means of the control slide and if a control slide surface causes actuation of the second supply control valve by moving past an actuating tappet connected to the second supply valve. The arrangement of the second supply control valve in relation to this control surface is such that the control surface of the actuating piston of the control slide comes into the region of the valve tappet only after any of the ASR control valve, the ASR outlet valve or the first supply control valve are changed over to their functional positions I assigned to ASR. Here the first supply control valve can be designed as a mechanically controlled 3/2-way valve which, as a result of the piston displacement (preferably of the secondary piston of the brake unit occurring during an actuation of the brake unit), is changed over from its basic position assigned to ASR into the functional position provided for the braking mode. This arrangement affords alternative, or joint actuation of supply control valves to allow pressure to be fed into the return pump of the brake circuit of the driven vehicle wheels during ASR.
A safety valve can be connected in parallel with the precharging pump. During an actuation of the brake system, this safety valve is moved out of its shut off basic position into its throughflow position, in which the control-pressure space of the control slide is connected directly to the pressureless brake-fluid reservoir of the brake system. The safety valve is designed as pressure-controlled 2/2-way valve which is activatable by means of the brake pressure generated in the secondary outlet-pressure space of the brake unit. This arrangement guarantees that the brake system responds sufficiently quickly if sudden braking has to take place in a driving situation in which the ASR has been activated and is especially advantageous in terms of the simplicity of its construction.
There is no need for a safety valve of this type if the precharging pump is designed as a gear pump wherein the leakage rate at standstill is very high. Here the control slide provided for actuating the ASR control valve by the outlet pressure of the precharging pump, is relieved of pressure sufficiently quickly when sudden braking has to take place while the ASR is being activated.
The precharging pump can be designed as a precharging further piston pump which is driveable by means of the drive provided for the return pumps and to provide for a simplified construction. This precharging pump should be a self-priming pump and be equipped with a displacement-controlled inlet valve which is moved into its open position when the pump piston is executing the final portion of its suction stroke and the initial portion of its delivery stroke, and which is otherwise shut off.
When a self-priming return pump is used as an ASR auxiliary-pressure source, there is no need for a precharging pump.
A fault detection device which responds to a signal from a first sensor output signal which is a measure of the displacement of a piston of a brake unit occurring as a result of an actuation of the brake system and to a second signal from a second sensor which is a measure of the brake-actuating force occurring as a result of the actuator, can be used to generate an indicator signal characteristic of the proper or the faulty functioning of the brake system. The second sensor output signal is generated by a pressure sensor which monitors the brake pressure occurring in one of the brake circuits of the vehicle. Preferably the brake circuit of the driven vehicle wheel is chosen. Here the brake unit is designed as a tandem master cylinder. The fault detection device comprises two displacement sensors, each monitoring the position of one of the two pistons of the tandem master cylinder and generating electrical output signals characteristic of the piston positions. As a result of a comparative processing of these output signals, the fault generator operates an indicator or warning signals characteristic of the proper or the faulty functioning of the brake system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.